A major or goal of all-optical processing is speed, which requires rapid access to high capacity, relatively short term memory. An optical memory is a basic element which is essential for more complex data processing, including pattern recognition applications.
The use of a fibre loop as a recirculating delay line, that is to say as an "optical memory", has been discussed for several years and demonstrated in conjunction with optical amplifiers. The potential high storage capacity of such a configuration was first realised in the all-optical, long distance soliton transmission experiments which employed distributed Raman amplification. A known optical memory configuration is shown in FIG. 1. Its basic elements consist of a long loop (25 km in this example) of single mode fibre 2, a fibre coupler 4 having a 3 dB coupling ratio, and a single erbium-doped fibre amplifier (EDFA) 6. The fibre is chosen to be suitable for soliton propagation over ultra-long fibre distances. To this end, the fibre used is dispersion-shifted, so that the dispersion is low, and soliton supporting (D.about.+1 ps/nm.km) at the operating wavelength. The signal pulses to be stored are then gated, via a first gate 8, into the loop 2 via a port 10 of the coupler 4, a 120 .mu.s pulse train just filling the 25 km long loop. As long as unity loop gain is maintained, the signal continues to circulate without decrease in pulse energy. A port 12 of the coupler 4 acts as a tap for the memory, and a second gate 14 selects the stored data after the desired time interval.
A disadvantage of this type of optical memory is that optical processing cannot be carried out on the stored data circulating in the loop, other than by gating in additional data pulses, or by erasing the memory and gating in a new stream of data pulses. It is an object of the present invention to provide an optical memory with improved optical processing capabilities.